- The growth of most medically important organisms is enhanced by the presence of oxygen, which acts as the hydrogen acceptor in the final stages of energy production catalyzed by the flavoproteins and cytochromes of the electron transport system.
- Organisms are often classified based on their ability to grow in the presence of oxygen:
- obligate aerobes: require oxygen for growth.
- microaerophiles: require oxygen at low concentration.
- obligate anaerobes: cannot grow in the presence of oxygen.
- aeroteolerant anaerobes: do not use oxygen, but can grow in its presence.
- facultative anaerobes: can grow in the presence or absence of oxygen.
- Aerobic growth produces 2 toxic molecules that can generate hydroxyl radicals which can damage proteins, DNA, lipids, etc. Cells have specific enzymes to deal with these toxic molecules:
- Superoxide (O2–) is converted to oxygen and hydrogen peroxide by the enzyme, superoxide dismutase: 2O2–Â +Â 2H+ —-Ă Â O2 + H2O2
- Hydrogen peroxide (H2O2) is broken down into water and oxygen by the enzyme, catalase: 2H2O2Â —-Ă Â 2H2O + O2
- Obligate anaerobes generally do not have catalase nor superoxide dismutase!
Photo by phylofigures 
Frequently Asked Questions
What is the difference between aerobic and anaerobic organisms?
Aerobic organisms require oxygen for growth and utilize it as a final electron acceptor in energy production. In contrast, anaerobic organisms do not require oxygen and may even be inhibited or killed by its presence. Anaerobic organisms rely on other molecules as electron acceptors to generate energy.
Why do obligate anaerobes struggle to survive in oxygen?
Obligate anaerobes lack the enzymes needed to neutralize harmful byproducts of oxygen metabolism, such as superoxide and hydrogen peroxide. Without enzymes like superoxide dismutase and catalase, these toxic molecules accumulate, causing damage to cellular components like proteins and DNA.
How do facultative anaerobes adapt to different oxygen levels?
Facultative anaerobes can grow in both the presence and absence of oxygen. They are versatile, switching between aerobic respiration when oxygen is available and anaerobic processes when it is not, allowing them to thrive in diverse environments.
What role do superoxide dismutase and catalase play in cells?
Superoxide dismutase and catalase are critical enzymes that protect cells from oxidative damage. Superoxide dismutase converts superoxide radicals into hydrogen peroxide, which catalase then breaks down into water and oxygen, preventing harmful oxidative stress.
What are microaerophiles and how do they differ from other aerobic organisms?
Microaerophiles require oxygen to survive but at much lower concentrations than what is found in the atmosphere. Unlike obligate aerobes, which need normal oxygen levels, microaerophiles thrive in environments where oxygen is limited, such as in the gastrointestinal tract.
Can aerotolerant anaerobes grow in oxygen-rich environments?
Yes, aerotolerant anaerobes can grow in the presence of oxygen, but they do not use it for their metabolic processes. They rely on fermentation or other anaerobic pathways for energy production and are equipped with mechanisms to tolerate oxygen.
Why is oxygen toxic to some organisms?
Oxygen can be toxic to some organisms because it can form reactive oxygen species (ROS) like superoxide radicals and hydrogen peroxide. These ROS can damage proteins, DNA, and lipids if not neutralized by specific enzymes, posing a threat to organisms lacking these protective mechanisms.
What are the energy production differences between aerobic and anaerobic metabolism?
Aerobic metabolism, involving oxygen, is much more efficient in energy production, yielding up to 38 ATP molecules per glucose molecule. Anaerobic metabolism, however, produces only 2 ATP per glucose due to the absence of oxygen as an electron acceptor, relying on processes like fermentation.